if doing so you have to remount the /system partition as it is mounted read only.
via an adb shell
mount -o remount,rw /system
or from outside the device
adb remount
After this you can copy the content of this file onto the system partition.
Keep in mind that these apk files will rely on some libraries, config files etc, which are also provided in this file and probably it would be best to install the whole as it would be done via the install method in recovery. If you really want to install only some of these apk files you have to figure out their dependencies and install them also. For analysing dependencies of an apk file you could use: http://developer.sonymobile.com/knowledge-base/tools/analyse-your-apks-with-apkanalyser/. The dependencies should be listed in the apks AndroidManifest.xml which you could also read with tools from the Android SDK like: aapt something like: aapt dump badging yourapp.apk.
See also: https://stackoverflow.com/questions/4191762/how-to-view-androidmanifest-xml-from-apk-file.
Keep in mind: iam not an app developer so iam not sure if libraries which should be part of the system/lib directory are also listed in it's AndroidManifest.xml file maybe an app developer could comment here.
Usually you should always prefer the installation through recovery as this is a safe way of installing where you can be sure not to break anything while installing, as the main system is not running.(Broken update.zip files can still break your device :P)
